1. Field of the Invention
The present invention relates to a method and apparatus for enriching oxygen in the heavy oxygen isotopes, 17O and 18O; and in particular, the present invention relates to a method and apparatus for enriching oxygen in these heavy oxygen isotopes by means of cryogenic distillation.
In addition, the present invention relates to a method and apparatus for further concentrating heavy oxygen isotopes by means of conducting isotope scrambling following the cryogenic distillation.
This application is based on patent application No. Hei 11-150733 filed in Japan, the content of which is incorporated herein by reference.
2. Background Art
Naturally abundant oxygen comprises 99.759% (atomic %, used in this way hereinafter) of 16O, 0.037% of 17O, and 0.204% of 18O.
Among these, the heavy isotope 18O is used as a tracer in fields such as agriculture, biology, and medicine.
In addition, in the same way, since the heavy isotope 17O has nuclear magnetic moment, it is used in the research of oxygen compounds using nuclear magnetic resonance and the like.
As enrichment methods for these heavy oxygen isotopes, there are distillation, thermal diffusion, chemical exchange (reactions), and the like. However, as a method of production with low cost and high volume, distillation is generally used. As the distillation method, there are methods which use water, NO, or CO as the starting material.
Among these method, as those methods whose success has been proven, water distillation methods using water as the starting material, and NO distillation methods using NO as the starting material can be mentioned.
As a water distillation method; the method practiced by Dostrovsky et al is known, and they reported that using this method it was possible to produce approximately 6 kg of 18O of a concentration of 98 to 99% in a year, and 1.5 kg of 17O of a concentration of 25% in a year. In addition, there are attempts to obtain high concentrations of 17O by means of further enrichment of 17O obtained by means of this method using a thermal diffusion method.
Since NO has a higher relative volatility compared with other starting materials, the enrichment efficiency for the above-mentioned isotopes in NO distillation methods is highly advantageous.
This method is used widely for enrichment of the isotopes of nitrogen and, normally, the above-mentioned heavy oxygen isotopes are obtained as bi-products of enrichment of the isotopes of nitrogen.
However, the above-mentioned conventional techniques have the following problems.
As shown in Table 1 and Table 2, since heavy isotopes are present in hydrogen and in nitrogen, there is the problem that in the above-mentioned water distillation methods, enrichment of water comprising the light isotope of oxygen (16O) and the heavy isotope of hydrogen occurs, and in NO distillation methods, enrichment of NO comprising the light isotope of oxygen (16O) and the heavy isotope of nitrogen occurs.
More specifically, in water distillation methods, it is easy for water containing the light isotope of oxygen (16O) and deuterium (HD16O, etc.) to become mixed into the obtained heavy isotope enriched product. This hinders enrichment of the H217O and H218O which contain the heavy isotopes of oxygen, and it is difficult to industrially obtain product which is highly enriched in the heavy isotopes of oxygen, such as H218O having a purity of 99% or greater. The purity of commercially available H218O is approximately 97%.
In addition, since the latent heat of vaporization of water is comparatively high (e.g., approximately six times that of the latent heat of vaporization of oxygen), the water distillation apparatus is comparatively large and energy consumption is great. For this reason, there is a tendency for water distillation methods to have increased apparatus and operational costs.
In addition, in NO distillation methods, it is easy for NO (15N16O) containing the heavy isotope of nitrogen and oxygen (16O) to become mixed into the obtained heavy isotope enriched product, and there is the problem that it is difficult to obtain an enriched product which is highly enriched in heavy oxygen isotopes.
In addition, due to reasons such as NO being a corrosive and poisonous gas, there is the problem that the above-mentioned NO distillation methods require a great deal of expense to put into practice.
In order to solve the above-mentioned problems, the method of the present invention provides a method of enrichment of heavy oxygen isotopes comprising enriching an oxygen starting material which contains heavy oxygen isotopes in at least one type of oxygen molecule selected from 16O17O, 16O18O, 17O17O, 17O18O and 18O18O, which contain heavy oxygen isotopes, by means of cryogenic distillation of the oxygen starting material which contains heavy oxygen isotopes.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes comprising enriching an oxygen starting material which contains heavy oxygen isotopes in at least one type of oxygen molecule selected from 16O17O, 16O18O, 17O17O, 17O18O and 18O18O, which contain heavy oxygen isotopes, by means of cryogenic distillation in which the oxygen starting material which contains heavy oxygen isotopes is supplied to a distillation column packed with structured packing.
In addition, in the method of enrichment of heavy oxygen isotopes of the present invention, as the method for the above-mentioned cryogenic distillation, a distillation method is used which comprises supplying an oxygen starting material to a distillation column which has been packed with structured packing; bringing about vapor-liquid contact between a descending liquid and an ascending vapor mainly on the surface of the above-mentioned structured packing within the above-mentioned distillation column; at which time, the liquid and the vapor flow in mutually opposite directions over the surface of the above-mentioned structured packing along the main flow direction, which is along the direction of the column axis, and at the same time mixing of the liquid and/or the vapor in a direction at right angles to the above-mentioned main flow direction is promoted and mass transfer occurs.
In addition, according to the present invention, it is preferable to perform the aforementioned cryogenic distillation of oxygen such that the density corrected superficial velocity (the superficial F factor) is at least 0.5 m/s(kg/m3)xc2xd and no greater than 2.0 m/s(kg/m3)xc2xd and more preferably, at least 0.8 m/s(kg/m3)xc2xd and no greater than 1.8 m/s(kg/m3)xc2xd.
In addition, according to the present invention, it is preferable to perform the aforementioned cryogenic distillation of oxygen such that the distillation pressure is in the range of 0.5 bar to 5 bar, and more preferably, 1.1 bar to 2.5 bar.
As the oxygen starting material, it is preferable to use highly pure oxygen having a purity of 99.999% or greater. In particular, it is preferable to use cryogenically manufactured high purity oxygen obtained from a high purity oxygen preparation device using cryogenic distillation.
In addition, the method of the present invention is a method for enrichment of heavy oxygen isotopes comprising using a distillation column comprising three distillation columns, a first column, a second column and a third column, as the above-mentioned distillation column; supplying an oxygen starting material from a feed section to the first column; supplying at least a part of the liquid or vapor output from the bottom of the first column to the second column; supplying at least a part of the liquid or vapor output from the second column to the third column; and extracting an enriched vapor having a concentration of 16O17O of 10% or greater from the top of the third column.
In addition, the method of the present invention is a method of enrichment of heavy oxygen isotopes comprising carrying out the distillation in such a way that in the second column a concentration peak of 16O17O is created at the middle of the column, and that a mixture of heavy oxygen isotopes comprising 16O17O at a concentration of 1% or greater, 16O18O at a concentration of 90% or greater, and the remainder being mostly 16O16O is separated at the bottom of the second column.
In addition, the method of the present invention is a method of enrichment of heavy oxygen isotopes comprising carrying out the distillation such that enriched liquid or vapor having a concentration of 16O18O of 90% or greater is separated at the bottom of the third column.
In addition, the method of the present invention is a method in which the distillation column is equipped with a condenser for cooling and liquefying vapor output from the distillation column, and a reboiler for heating and vaporizing the liquid output from the distillation column, and a medium for heat exchange for exchanging heat with the output vapor and the output liquid in the condenser and the reboiler, wherein at least one gas selected from nitrogen, oxygen, air, and the exhaust gas of an air separation unit is used as the medium for heat exchange.
In addition, it is preferable if a structure comprising a plurality of distillations columns is used as the above-mentioned distillation column.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes comprising enrichment of at least one type of oxygen molecule selected from among 16O17O, 16O18O, 17O17O, 17O18O and 18O18O, which contain heavy oxygen isotopes, by means of performing cryogenic distillation of an oxygen starting material containing heavy oxygen isotopes; subsequently conducting isotope scrambling; and obtaining an enriched product comprising a high concentration of at least one type of oxygen molecule containing the above-mentioned heavy oxygen isotopes.
The above-mentioned xe2x80x9cisotope scramblingxe2x80x9d is a general term describing the phenomena where in the presence of a plurality of molecular isotopes, each molecule randomly exchanges atoms with the other molecules. The xe2x80x9cisotope exchangexe2x80x9d using a catalyst described below is a typical example of this.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein an enriched product comprising an even higher concentration of at least one type of oxygen molecule containing said heavy oxygen isotopes is obtained by means of performing repeat cryogenic distillation on a heavy oxygen isotope enriched material obtained by means of the above-mentioned isotope scrambling.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the concentration of:at least one component of a heavy oxygen isotope enriched material, obtained by means of the aforementioned method of enrichment of heavy oxygen isotopes, is increased by means of conducting additional isotope scrambling.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the concentration of at least the heavy isotope oxygen 18O18O is further increased by means of performing repeat cryogenic distillation on the heavy oxygen isotope enriched material obtained by means of the aforementioned method.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein a heavy oxygen isotope enriched material containing an increased concentration of the heavy isotope oxygen 18O18O, and an enriched product containing an increased concentration of the heavy oxygen isotope 17O are obtained by means of performing further cryogenic distillation on a heavy oxygen isotope enriched material obtained by means of the aforementioned method.
In addition, the present invention provides a method of enrichment of heavy oxygen isotopes wherein a plurality of distillation columns are used and operated such that the maximum concentration of 17O18O appears in the middle section within the penultimate (next-to-last) distillation column, at the time of carrying out the enrichment of oxygen molecules containing heavy oxygen isotopes by means of performing the aforementioned cryogenic distillation.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the aforementioned isotope scrambling for concentrating the above-mentioned heavy oxygen isotopes comprises isotope exchange using a catalytic reaction.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the aforementioned: isotope scrambling for concentrating the above-mentioned heavy oxygen isotopes comprises adding hydrogen to and reacting it with the above-mentioned heavy oxygen isotope enriched material to produce water containing a high concentration of the above-mentioned heavy oxygen isotopes; and subsequently conducting electrolysis of said produced water to separate it into oxygen containing heavy oxygen isotopes and hydrogen.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the aforementioned isotope scrambling for concentrating the heavy oxygen isotopes comprises passing the above-mentioned heavy oxygen isotope enriched material through plasma by means of silent discharge, high-frequency discharge, or electromagnetic induction.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the aforementioned isotope scrambling for concentrating the heavy oxygen isotopes comprises irradiating the above-mentioned heavy oxygen isotope enriched material with ultraviolet rays to form ozone from said enriched material, and then decomposing the ozone.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the aforementioned isotope scrambling for concentrating the heavy oxygen isotopes is carded out by means of an oxidation-reduction reaction of the above-mentioned heavy oxygen isotope enriched material with BaO, SrO, CaO, Cu2O, FeO, CO, Mn3O4, Ag, Au, and/or a mixture thereof.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the aforementioned isotope scrambling for concentrating the heavy oxygen isotopes comprises isotope exchange in which the above-mentioned heavy oxygen isotope enriched material is thermally treated at a temperature of 1000xc2x0 C. or higher.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the above-mentioned reaction between the above-mentioned enriched material and hydrogen is conducted by means of combustion using a combustion chamber.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein the above-mentioned reaction between the above-mentioned enriched material and hydrogen is a catalytic reaction in which an inert gas such as Ar is supplied to said reaction system as a diluent gas to dilute the above-mentioned enriched material and hydrogen.
In addition, according to the present invention, examples of the catalyst used for catalytic reaction of the above-mentioned enriched material and the above-mentioned hydrogen may include a catalyst containing at least one component selected from the group comprising Pd, Pt, Rh, Ru, Ni, Cu, Au, Mn and metal oxides thereof
Furthermore, it is possible to use at least one type of catalyst in which these metals or metal oxides (Pt, Pd, Rh, etc.) are carried by Al-oxide, Si-oxide, Ti-oxide, Zr-oxide, Cr-oxide, V-oxide, Co-oxide, Mn-oxide, and the like.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein water produced by means of reacting the above-mentioned enriched material with hydrogen is cooled and condensed; said condensed water is separated from the diluent gas; and the diluent gas separated from the condensed water is returned to the above-mentioned reaction system for recirculation and reuse.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein hydrogen produced by means of the above-mentioned electrolysis is recycled and reused as hydrogen for addition to the above-mentioned enriched material.
Additionally, the present invention provides a method of enrichment of heavy oxygen isotopes wherein impurities in oxygen produced by means of the above-mentioned electrolysis are removed through oxidization by means of a catalytic reaction.
As the catalyst used for said catalytic reaction, a catalyst comprising at least one component selected from among the group comprising Pt, Pd, Rh, Ru, Au, Ni, Cu, and Agxe2x80x94Pd is suitable.
Further, more preferred examples include at least one type of catalyst wherein these metals (Pt, Pd, Rh, etc.) are carried by one of the aforementioned metallic oxides (Al-oxide, Si-oxide, etc.).
The apparatus used in the present invention is an apparatus for the enrichment of heavy oxygen isotopes which comprises a distillation column packed with structured packing and is an apparatus for enriching at least one type of oxygen molecule selected from among 16O17O, 16O18O, 17O17O, 17O18O and 18O18O which contain heavy oxygen isotopes by means of cryogenic distillation of oxygen.
In the same way, in the apparatus for the enrichment of heavy oxygen isotopes of the present invention, the above-mentioned structured packing is promoting-fluid-dispersion type structured packing which has a structure such that when a liquid descending in the distillation column and a vapor ascending in the distillation column make contact, the liquid and the vapor flow in mutually opposite directions over the surface of the above-mentioned structured packing along the main flow direction, which is along the direction of the column axis, and at the same time mixing of the liquid and/or the vapor in a direction at right angles to the above-mentioned main flow direction is promoted and mass transfer occurs.
In addition, in the apparatus for the enrichment of heavy oxygen isotopes of the present invention, the specific surface area of the packing in the above-mentioned distillation column is in the range of 350 m2/m3 to 1200 m2/m3, and preferably 500 m2/m3 to 750 m2/m3.
The apparatus for the enrichment of heavy oxygen isotopes of the present invention comprises at least one distillation column for enriching at least one type of oxygen molecule selected from among 16O17O, 16O18O, 17O17O, 17O18O and 18O18O, which contain heavy oxygen isotopes, by means of cryogenic distillation of an oxygen starting material containing heavy oxygen isotopes; and at least one isotope scrambler for increasing the concentration of at least one type of oxygen molecule selected from among 16O17O, 16O18O, 17O17O, 17O18O and 18O18O, which contain heavy oxygen isotopes, in the heavy oxygen isotope enriched material obtained from the above-mentioned distillation column, by means of isotope scrambling.
Additionally, in the apparatus for the enrichment of heavy oxygen isotopes of the present invention, the above-mentioned isotope scrambler is provided with an isotope exchange catalyst for the promotion of isotope exchange in the above-mentioned enriched material, and this isotope exchange catalyst includes at least one constituent selected from among the group comprising W, Ta, Pd, Rh, Pt and Au.
Additionally, in the apparatus for the enrichment of heavy oxygen isotopes of the present invention, the above-mentioned isotope scrambler is provided with an isotope exchange catalyst for the promotion of isotope exchange in the above-mentioned enriched material, and this isotope exchange catalyst includes at least one constituent selected from among the group comprising Ti-oxide, Zr-oxide, Cr-oxide, Mn-oxide, Fe-oxide, Co-oxide, Ni-oxide, Cu-oxide, Al-oxide, Si-oxide, Sn-oxide, and V-oxide.
In addition, in the apparatus for the enrichment of heavy oxygen isotopes according to the present invention, the distillation column is packed with structured packing, and the above-mentioned structured packing is promoting-fluid-dispersion type structured packing which has a structure such that when a liquid descending in the distillation column and a vapor ascending in the distillation column make contact, the liquid and the vapor flow in mutually opposite directions over the surface of the above-mentioned structured packing along the main flow direction, which is along the direction. of the column axis, and at the same time mixing of the liquid and/or the vapor in a direction at right angles to the above-mentioned main flow direction is promoted and mass transfer occurs.
In addition, in the apparatus of the present invention, the specific surface area of the packing in the above-mentioned distillation column may be in the range of 350 m2/m3 to 1200 m2/m3, and preferably 500 m2/m3 to 750 m2/m3.
Additionally, in the apparatus of the present invention, the above-mentioned distillation column may comprise a plurality (n) of distillation columns (Alxcx9cAn), wherein the bottoms of the columns Ak (k: a natural number of (nxe2x88x921) or less) are connected to the tops of columns Ak+1 by a conduit pipe via a liquid transfer means which sends liquid output from column Ak to column Ak+1, and the lower part of the column Ak is connected to the top of column Ak+1 by a conduit pipe for transferring the vapor output from the column Ak+1 to the column Ak.
Additionally, in the apparatus of the present invention, a condenser is preferably provided at the top of the aforementioned column A1, and a reboiler is preferably provided at the bottom of the aforementioned column Ak+1.
Additionally, in the present invention, a circulation system for a medium for heat exchange which connects a second conduit of the above-mentioned condenser and a second conduit of the above-mentioned reboiler may be provided, and a circulation means for circulating the medium for heat exchange (for example, air, nitrogen, oxygen, or the like) may be provided somewhere along the above-mentioned circulation system.
The aforementioned circulation means may comprise a low-temperature compressor.
In addition, the aforementioned circulation means may comprise a normal temperature compressor. In this case, a heat exchanger for conducting heat exchange between the medium for heat exchange at the inlet of the above-mentioned normal temperature compressor and the medium for heat exchange at the outlet of the above-mentioned normal temperature compressor is preferably provided.
In addition, in the apparatus of the present invention, a plate fin type condenser maybe provided at the top of the above-mentioned distillation column, and a coil-type reboiler or a plate fin type reboiler may be provided within the above-mentioned column in the vicinity of the bottom. In addition, a conduit pipe is connected to the inlet side of a first conduit of the above-mentioned condenser for introducing at least a part of the output vapor from the top of the distillation column into the above-mentioned first conduit of the above-mentioned condenser. A conduit pipe is connected to the outlet side of the first conduit for introducing liquid output from this conduit into the top of the above-mentioned distillation column again. A conduit pipe for circulating a medium for heat exchange is connected to the second conduit of the condenser, this conduit pipe for circulating a medium for heat exchange is connected to the above-mentioned coil type reboiler or plate fin type reboiler, and a circulation means is provided in the above-mentioned conduit pipe for circulating the medium for heat exchange within the above-mentioned conduit pipe.